1. Field of the Invention
The present invention relates to a method of AC driving a display unit made of display material such as liquid crystal by using an active matrix constructed of switching elements such as thin film transistors (hereinafter called TFT) and pixel electrodes, and also to a method of setting its drive voltages, respectively aiming at a) reduction of drive power, b) improvement on display quality, and c) improvement on drive reliability.
2. Description of Prior Art
The display quality of active matrix display units has been considerably improved nowadays, to the degree that it stands unrivaled by CRTs. However, first, from the view point of image quality, it cannot be said that active matrix display units are as good as CRTs with respect to flickers; brightness change on a screen in a vertical direction, i.e., brightness inclination; image memory phenomenon such that after a fixed image is displayed, it remains on the screen as if it has been burnt on it; gradation display performance; and the like. There have not been reported as yet techniques to radically solve the adverse effects of DC voltage and crosstalk which are unavoidably generated by parasitic capacitors within a display unit.
The following techniques are known for the improvement of flickers. There is disclosed in Japanese Patent Laid-open Publications JP-A-60-151615, JP-A-61-256325, and JP-A-61-275823, the technique that the polarities of signal voltages are reversed alternately for each display field. There is disclosed in Japanese Patent Laid-open Publications JP-A-60-3698, JP-A-60-156095, and JP-A-61-275822, the technique that the polarities of signal voltages are reversed alternately for each scan line. There is disclosed in Japanese Patent Laid-open Publication JP-A-61-275824 the technique that the polarities of signal voltages are reversed alternately both for each display field and for each scan line.
With the above techniques, no compensation is provided for the DC voltage (described below) which is unavoidably generated due to the dielectric anisotropy of display material such as liquid crystal, due to parasitic capacitors in a display unit, or due to other causes, and flickers are conventionally intended to be reduced not for each pixel but apparently and collectively for all pixels.
There is also known a technique which intends to reduce crosstalk in a particular active matrix arrangement, as disclosed in "Euro Display" by K. Oki et al., '87, p 55. In this technique, a reference signal is added before a scan signal to thereby reduce an image signal amplitude and hence crosstalk. Another known crosstalk reduction technique is disclosed in "International Display Research Conference (I.D.R.C.)" by W.E. Howard et al, '88, p. 230. This technique intends to compensate for crosstalk voltage after the application of an image signal. The above two techniques do not compensate for the DC voltage of liquid crystal as will be described later.
There is not known a technique which intends to improve the brightness inclination and gradation display performance per se.
There are known the following two documents which disclose the technique of compensating for the DC voltage unavoidably generated in a display unit due to dielectric anisotropy of liquid crystal, of radically reducing flicker, and of improving drive reliability. The first document "JAPAN DISPLAY" by T. Yanagisawa, et al, '86, p. 192 intends to compensate for the DC voltage by using an image signal voltage (Vsig) having different positive and negative amplitudes relative to a base or center voltage (Vc). The second document "Euro Display" by K. Suzuki, '87, p. 107 intends to compensate for the DC voltage by adding a negative signal (Ve) after a scan signal.
The third problem is that a DC potential difference occurs between the average potential at an image signal line and that at a pixel electrode because a scan signal adversely effects the pixel electrode potential via a parasitic capacitor Cgd between the gate and drain of a TFT. If the potentials at various circuit portions of a display unit are set so as to make zero the average DC potential difference between the pixel electrodes and opposing electrode during the AC drive of liquid crystal, there is unavoidably generated the DC potential difference between the image signal line and opposing electrode. This DC potential difference results in a serious defect such as the image memory phenomenon. There is not known, however, a method of compensating for such DC potential difference.
The fourth problem is that contrary to the characteristic feature of small drive power of a liquid crystal display unit, in an actual case, the conventional drive circuit processes analog signals by using a great number of signal output circuits so that it consumes a large power (several hundreds mW) which is not suitable for operating it with a battery or the like in a portable apparatus. It has therefore been desired to develop a method of driving a display unit with low power consumption.